A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation source for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g., tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The laser beam that is directed at the fuel may be an Infra Red (IR) laser (i.e., a laser that emits radiation at an IR wavelength), such as a Carbon Dioxide (CO2) laser or a Yttrium Aluminium Garnet (YAG) laser. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
As discussed above, within LPP sources radiation is directed at a fuel. The properties of the radiation that is output by the radiation producing plasma depend upon the alignment between the fuel and the focus of the radiation that is directed at the fuel. For example, two properties of the radiation output by the radiation producing plasma that are affected by the alignment between the fuel and the focus of the radiation directed at the fuel are the total intensity and the intensity distribution of the radiation output by the radiation producing plasma. It will be appreciated that in certain applications of the radiation source it is beneficial for the intensity distribution of the radiation output by the radiation producing plasma to be substantially uniform. Furthermore, certain lithographic apparatus may require a particular intensity distribution of radiation is produced by the radiation source and it is desirable that such an intensity distribution is reproducible. Moreover, in some applications it is desirable to for the radiation producing plasma to output the maximum possible total intensity of radiation. For these reasons at least, it is desirable to have some indication of the relative alignment between the fuel and the focus of the radiation directed at the fuel.
The ability to have some indication of the relative alignment between the fuel and the focus of the radiation directed at the fuel may be beneficial due to the fact that it may be desirable to control the LPP source so that the radiation output from the radiation source has a desired distribution. Alternatively, or in addition, it may be desirable to have an indication of the relative alignment between the fuel and the focus of the radiation directed at the fuel due to the fact that both the position of the fuel and the position of the focus of the radiation directed at the fuel may be subject to external disturbances. For example, the focus position of the radiation directed at the fuel and the position of the fuel (and hence the alignment between the fuel and the focus of the radiation directed at the fuel) may be affected by system dynamics of the lithographic apparatus, such as the movement of parts of the lithographic apparatus. The ability to have an indication of the relative alignment between the fuel and the focus of the radiation directed at the fuel means that any misalignment between the fuel and the focus of the radiation directed at the fuel can be corrected.
In some known lithographic apparatus, the relative alignment between the fuel and the focus of the radiation directed at the fuel is measured indirectly. For example, a sensor referred to as a quad sensor may be used to measure the intensity distribution of the radiation output by the radiation producing plasma. By measuring the intensity distribution of the radiation output by the radiation producing plasma, it is possible to infer information about the relative alignment between the fuel (that produced the radiation producing plasma) and the focus of the radiation directed at the fuel. The quad sensor has a plurality of sensor elements (for example 3 or 4), which are located within the radiation source and that may, for example, be equi-angularly spaced about an optical axis of the radiation output by the radiation producing plasma. By measuring the intensity of the radiation output by the radiation producing plasma, which is incident on each sensor element, it is possible to determine the intensity distribution of the radiation output by the radiation producing plasma. As previously discussed, by measuring the intensity distribution of the radiation output by the radiation producing plasma, it is possible to infer information about the relative alignment between the fuel and the focus of the radiation directed at the fuel. This information relating to the relative alignment between the fuel and the focus of the radiation directed at the fuel may be used to correct any misalignment between the fuel and the focus of the radiation directed at the fuel.
There are various problems associated with this method of determining information about the relative alignment between the fuel and the focus of the radiation directed at the fuel. These problems are discussed below.
First, due to the fact that information about the relative alignment between the fuel and the focus of the radiation directed at the fuel is obtained by measuring properties of the radiation output by the radiation producing plasma, the determination of the information concerning the alignment between the fuel and the focus of the radiation directed at the fuel is dependent upon the interaction between the fuel and the radiation, which is incident on the fuel, as well as the physics governing the generation of the radiation producing plasma and the physics governing the nature of the plasma itself.
The specifics of the interaction between the fuel and the radiation incident on the fuel, and also the properties of the radiation producing plasma, are not well known. For this reason, it is not possible to predict with absolute certainty what the alignment between the fuel and the focus of the radiation directed at the fuel based on measuring properties of the radiation output by the radiation producing plasma. Furthermore, due to the properties of the radiation producing plasma, for any given alignment between the fuel and the focus of the radiation directed at the fuel, the measured intensity/intensity distribution of the radiation output by the radiation producing plasma may be time-varying. Furthermore, the relationship between the alignment between the fuel and the focus of the radiation directed at the fuel and the measured intensity/intensity distribution of the radiation output by the radiation producing plasma may be non-linear. For this reason, measuring the properties of the radiation output by the radiation producing plasma makes it difficult to predict the relative alignment between the fuel and the focus of the radiation directed at the fuel with a high degree of accuracy. Moreover, properties of the radiation that is directed at the fuel may not be constant. Consequently, the measured properties of the radiation output by the radiation producing plasma may vary due to factors that are independent of the relative alignment between the fuel and the focus of the radiation directed at the fuel (such as changing properties of the radiation directed at the fuel).
The lack of accuracy in being able to determine the relative alignment between the fuel and the focus of the radiation directed at the fuel may make such a system for determining the relative alignment between the focus and the fuel unsuitable for high bandwidth control (i.e., control loops that operate at a high frequency).
Secondly, determining information regarding the relative alignment between the fuel and the focus of the radiation directed at the fuel by measuring a property of the radiation output by the radiation producing plasma requires that the radiation producing plasma is producing radiation, a property of which can be measured. When there is no output radiation being generated by the plasma (for example, if the fuel has not yet had radiation incident upon it) then it will not be possible to measure any property of radiation output by the radiation producing plasma, and as such it will not be possible to infer any information about the relative alignment between the fuel and the focus of the radiation directed at the fuel. This may lead to additional start-up and/or recovery time for a lithographic apparatus, which includes a radiation source that operates in this manner. Any additional start-up and/or recovery time of the lithographic apparatus is time in which the lithographic apparatus is not producing a product, and hence this reduces the output efficiency of the lithographic apparatus.
Thirdly, the sensing elements of the quad sensor that is used to measure properties of the radiation output of the radiation producing plasma are exposed to the radiation output by the radiation producing plasma. This may be disadvantageous in a situation whereby the radiation output by the radiation producing plasma is detrimental to the sensing elements of the quad sensor. For example, in the case where the radiation output by the radiation producing plasma is EUV radiation, the EUV radiation may damage the sensing elements of the quad sensor over time, thereby causing the quad sensor to degrade. Furthermore, the quad sensor may be located at a position that is in proximity to the radiation producing plasma. Consequently the quad sensor may be exposed to heat and/or debris from the radiation producing plasma that may damage or degrade the quad sensor. The damage or degradation of the quad sensor over time may cause the sensing characteristics of the quad sensor to vary over time such that the output of the quad sensor becomes either inaccurate or incapable of producing useful information about the relative alignment between the fuel and the focus of the radiation directed at the fuel. Furthermore, in extreme circumstances, the quad sensor may be damaged or degrade to the extent that it is no longer operable.
Some lithographic apparatus may have a radiation source that functions differently to that previously described. In these cases, the production of output radiation from a fuel is a two-step process. The first step is that a first pulse of radiation is directed at the fuel such that the first amount of radiation is incident on the fuel, and converts the fuel into a modified fuel distribution. For example, the modified fuel distribution may be a cloud of partially plasmarised fuel. A subsequent amount of radiation may be directed at the modified fuel distribution such that the subsequent amount of radiation is incident on the modified fuel distribution, causing the modified fuel distribution to become a radiation producing plasma, which outputs the desired radiation.
The first amount of radiation incident on the fuel may be referred to as a pre-pulse and the subsequent amount of radiation incident on the modified fuel distribution may be referred to as a main-pulse.
In cases involving a pre-pulse and a main-pulse, both the relative alignment between the focus of the pre-pulse and the fuel, and the relative alignment between the focus of the main-pulse and the modified fuel distribution may be important in determining properties of the radiation output by the radiation producing plasma (for example, the intensity or the intensity distribution of the radiation output by the radiation producing plasma). Furthermore, it is thought that because the size of the fuel upon which the pre-pulse is incident is small compared to the size of the modified fuel distribution upon which the main-pulse is incident, it is likely that it is the relative alignment between the focus of the pre-pulse and the fuel that will be more critical than the alignment between the focus of the main-pulse and the modified fuel distribution to the properties of the radiation that is output by the radiation producing plasma.
However, as previously discussed, due to the fact that the pre-pulse radiation that is incident on the fuel will not create a radiation producing plasma, very little or no radiation will be produced as a result of the pre-pulse being incident on the fuel. Consequently, very little or no radiation will be measured by the quad sensor and hence the quad sensor may not be capable of providing any information about the relative alignment between the focus of the pre-pulse and the fuel. Furthermore, due to the fact that the properties of the modified fuel distribution are not well understood, it may not be possible to determine information about the relative alignment between the focus of the main-pulse and the modified fuel distribution by measuring the intensity distribution of the output radiation produced by the radiation producing plasma.
It is desirable to provide a radiation source that obviates or mitigates at least one of the problems of the prior art whether described above or otherwise. It is also desirable to provide an alternative radiation source.